It is often desirable to rapidly and accurately deflect an electron or ion beam. In beam systems, the beam is necessarily surrounded by equipment for maintaining a vacuum, as well as shaping, deflecting and focusing the beam. In high speed submicron lithography, an electron beam is imaged toward the target. In the class of structures considered herein, the final lens is a large bore lens having a pair of saddle coils positioned therein for deflecting the beam on the target.
The accuracy of a magnetic deflection system is determined in part by the degree to which magnetic field interactions with surrounding conductors are avoided or compensated. These interactions produce eddy currents and thus parasitic magnetic fields. The parasitic magnetic fields cause temporary spot displacements.
Two types of eddy currents can be distinguished, and neither can be tolerated at an uncorrected level if deflection accuracies near 20,000 lines per field are desired. The first type of eddy current results from interactions between the deflection field and surrounding conductors. The most massive conductor is usually the final lens iron yoke. Eddy currents in this yoke result in fields which can produce deflection short-fall in the range of 0.1 to 1% (of the desired deflection distance) and which typically lasts 10 to 100 milliseconds. Eddy currents of this type are referred to herein as "outer" eddy currents. G. A. Wardly considered these outer eddy currents in his article in Journal of Applied Physics, Vol. 44, No. 8, August 1973, pp. 3766-3769, in his article in Journal of Applied Physics, Vol. 44, No. 12, December 1973, at pp. 5607-5612, and in his article in Journal of Applied Physics, Vol. 45, No. 5, May 1974, at pp. 2316-2320.
It has been determined that there is a second eddy current phenomena in the skin effect in the deflection coils. It arises from interactions between the deflection field and wires of the deflection coil. Accordingly, there is need for a means by which this second eddy current phenomena can be compensated.